Friction stir welding of metal matrix composites

ABSTRACT

A method of making a fiber-reinforced component includes: providing a first composite member having a metal matrix with reinforcing fibers distributed therein; providing a second composite member having a metal matrix with reinforcing fibers distributed therein; and joining the first member to the second member by friction stir welding along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in the composite members before joining. A fiber-reinforced component includes: first and second members each having a metal matrix with reinforcing fibers distributed therein. The first member is bonded to the second member by a solid state bond along a predetermined joint path, such that an average volume fraction of the reinforcing fibers within the joint path is substantially the same as an average volume fraction thereof in remainder of the members.

BACKGROUND OF THE INVENTION

This invention relates generally to metal matrix components and moreparticularly bonding of such components by friction stir welding.

It is known in the prior art to construct composite materials using ametallic matrix with reinforcing fibers, hereinafter referred to as“metal matrix composites”. These materials combine light weight and goodstrength. Typically, the reinforcing fibers are relatively short inlength and are oriented randomly so that the component will haveisotropic properties. Non-limiting examples of turbine engine componentswhich may be constructed from such composites include rotating fanblades and other kinds of airfoils, rotating shafts and disks, staticstructures.

Metal matrix composites can be molded to desired shapes or can be bondedthrough means such as heat welding. Unfortunately, the fluid flow thatoccurs during the welding process disturbs this intended orientation andtherefore undesirably creates an area along the joint in which only thematrix carries any loads placed on the component.

Accordingly, there is a need for joining metal matrix composites whilemaintaining their mechanical properties.

BRIEF SUMMARY OF THE INVENTION

The above-mentioned need is met by the present invention, whichaccording to one aspect provides a method of making a fiber-reinforcedcomponent, including; providing a first composite member comprising ametal matrix with reinforcing fibers distributed therein in a selectedorientation; providing a second composite member comprising a metalmatrix with reinforcing fibers distributed therein in a selectedorientation; and joining the first member to the second member byfriction stir welding along a predetermined joint path, such that anaverage volume fraction of the reinforcing fibers within the joint pathis substantially the same as an average volume fraction thereof in thecomposite members before joining.

According to another aspect of the invention, a fiber-reinforcedcomponent includes: a first composite member comprising a metal matrixwith reinforcing fibers distributed therein in a selected orientation;and a second composite member comprising a metal matrix with reinforcingfibers distributed therein in a selected orientation; wherein the firstmember is bonded to the second member by a solid state bond along apredetermined joint path, such that an average volume fraction of thereinforcing fibers within the joint path is substantially the same as anaverage volume fraction thereof in remainder of the members.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be best understood by reference to the followingdescription taken in conjunction with the accompanying drawing figuresin which:

FIG. 1 is top plan view of a prior art component comprising twothermally bonded members;

FIG. 2 is a side view of two members to be joined using a friction stirwelding process;

FIG. 3 is a top view of two members being joined with a friction stirwelding process;

FIG. 4 is a top view of the members of FIG. 3 after a friction stirwelding process; and

FIG. 5 is top view of the members of FIG. 3 after bonding by analternative friction stir welding process.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein identical reference numerals denotethe same elements throughout the various views, FIG. 1 depicts anexemplary prior art reinforced metal matrix component 10 comprisingfirst and second members 12 and 14 bonded together along a joint path16. The members 12 and 14 are both made from a composite materialcomprising a metal polymeric matrix 18 with reinforcing fibers 20disposed therein. In the illustrated example the reinforcing fibers 20have a random three-dimensional orientation to impart isotropicstructural properties to the members 12 and 14. It should be noted thatthe fibers 20 are depicted in highly exaggerated scale for the purposeof illustration. The members 12 and 14 are bonded together using aconventional process such as thermal welding wherein the matrix 18 ofeach member 12 and 14 is heated along the joint, to a temperature aboveits solidus point, so that it will melt and flow together. The members12 and 14 are then allowed to cool to form the unitary component 10.Unfortunately, the thermal welding process creates a heat-affected zone22 within which the reinforcing fibers 20 are absent or disturbed intheir density of distribution, or orientation. Under thesecircumstances, the area around the joint path 16 lacks the full strengththat would be expected from the use of the reinforcing fibers 20.

FIGS. 2-4 depict a process of bonding reinforced metal matrix componentstogether in a butt joint using friction stir welding. In this example,first and second members 112 and 114 are bonded together along a jointpath 116 to form a completed component 110. The illustrated members 112and 114 are simple plate-type elements with a constant thickness.However, these are merely representative examples, and the processdescribed herein may be used to join any type of component which isamenable to friction stir welding. Examples of turbine engine componentswhich may be constructed from fiber reinforced metals include rotatingfan blades, outlet guide vanes, reverser cascades, and various otherstatic structures. Furthermore, the present method is applicable tojoint configurations other than the illustrated butt joint.

Each of the first and second members 112 and 114 comprise a metal matrix118 with reinforcing fibers 120 disposed therein. In the illustratedexample the reinforcing fibers 120 have a random three-dimensionalorientation to impart isotropic structural properties to the members 112and 114, but other orientations could be used to achieve desiredproperties. The reinforcing fibers 120 are essentially uniformlydistributed throughout the volume of each of the members 112 and 114.This distribution can be described as an average volume fraction offibers for a unit volume of the matrix 118, i.e. a value of 0.0 wouldrepresent a total lack of reinforcing fibers 120 within the matrix 118,and a value of 1.0 would represent a solid mass of reinforcing fibers120.

The metal matrix 118 will vary depending on the requirements of thespecific application. One non-limiting example of a metal known to besuitable for structural components is titanium and alloys thereof.

The reinforcing fibers 120 will also vary according to the specificapplication. The fibers beneficially will have a tensile strengthgreater than that of the matrix 118 in order to form a synergisticstructural combination with the matrix 118. Non-limiting examples ofmaterials useful for reinforcing fibers 120 include silicon coatedcarbon, silicon carbide, tungsten, glass, other kinds of carbon fibers,and metals. In the illustrated example, the reinforcing fibers 120 havea diameter of about 1 micrometer (40 microinches) to about 25micrometers (980 microinches), with aspect ratios of about 100 to about15,000 with resultant lengths of about 1 mm (0.004 in.) to about 38 cm(14.7 in.)

The member 112 is joined to the member 114 using a friction stir weldingprocess. The welding process is carried out using friction stir weldingmachinery and fixtures of a known type (not shown). As shown in FIG. 2,a cylindrical, shouldered, wear-resistant pin “P” having a tip “R” isrotated and forced against the joint path 116. The friction between thepin P and the members 112 and 114 causes the material to soften and flowwithout melting. Thus, friction stir welding is a type of solid statebonding process. In the illustrated example the pin P has a shoulderdiameter “D” of about 10.7 mm (0.420 in.), and the tip R has a length“l” of about 2.8 mm (0.110 in.) from the shoulder to its distal end anda diameter “d” of about 6.4 mm (0.250 in.), and has a left-hand threadformed thereon. The following exemplary parameters have been found toproduce an acceptable friction stir welded bond: pin speed about 700 toabout 900 RPM; traversing speed about 10 cm/min. (4 in/min.) to about15.2 cm/min. (6 in/min.); and force on the pin P about 499 kg (1100lbs.) to about 635 kg (1400 lbs.). The pin P is traversed along thejoint path 116, straddling the member 112 and 114, leaving the members112 and 114 bonded together behind it.

As the pin P is traversed along the joint line, the heat generated isconducted away from the pin P and to the surface of the members 112 and114, which results in a decreasing temperature gradient. Along thisgradient, various zones can be identified according to the effect on themembers 112 and 114. A stir zone “S” is created which has a widthslightly greater than the width of the tip R, for example about 0.25 mm(0.010 in.) from the edge of the tip R on each side. Athermomechanically altered zone “T” extends outward from the edge of thestir zone “S”, for example about 0.25 mm (0.010 in.) on each side. Aheat affected zone “H” extends outward from the edge of thethermomechanically affected zone T, for example about 0.76 mm (0.030in.) on each side. The width of each of these zones will be affected bythe thermal properties of the members 112 and 114, as well as theirshape and dimensions.

Within the stir zone S, a vortex spiral circular flow of the matrix 118is generated around the tip R. Because the matrix 118 is in a fluidstate, the reinforcing fibers 120 are free to move with this flow. Theyare carried around the periphery of the tip R (see FIG. 3). It has beenfound that reinforcing fibers 120 will tend to align their longitudinalaxes parallel to the shear gradient in the material. Thus, they willtend to remain tangent to the vortex flow as they are carried around it.Within the thermomechanically altered zone T, there is reduced transportof the reinforcing fibers 120, but they tend to orient themselvesparallel to a moving shear plane normal to the joint path 116. Incontrast to prior art types of thermal bonding, the reinforcing fibers120 will tend to remain within in the vicinity of the joint path 116.

As the probe P traverses the joint line, the stir zone S cools andsolidifies, resulting in consolidation between the member 112 and themember 114. Individual fibers 120 will remain in the locations andorientations where the matrix 118 “traps” them during solidification.The friction stir welding parameters can be modified to influence thefinal orientation of the reinforcing fibers 120. For example, thetraversing speed can be increased or decreased relative to the pinspeed. A relatively rapid traverse rate will tend to result in reducedtransport of the reinforcing fibers 120 across the joint path 116, whilea relatively higher traverse rate will result in increased transport ofthe reinforcing fibers 120 across the joint path 116. Furthermore,higher pin speed or pressure will increase the size of the stir zone Sand the thermomechanically altered zone T, tending to increase theamount of transport. FIG. 4 shows the component 110 bonded in such a waythat the reinforcing fibers 120 are substantially randomly orientedalong the joint path 116. FIG. 5 shows a similar component 110′ madefrom two members 112′ and 114′ bonded along a joint path 116′. In thisexample, the reinforcing fibers 120′ are more likely to be orientedtransverse to the joint path 116′. Such an orientation might be expectedfrom using a relatively low traverse rate.

The completed weld leaves a smooth surface finish along the joint pathwhich requires minimal processing to result in an acceptable finishedproduct. In contrast to prior art thermal bonding methods, there will bea significant distribution of reinforcing fibers 120 within and acrossthe joint path 116, similar to the average fiber volume fraction beforebonding. Accordingly, the structural properties of the members 112 and114 are substantially preserved, and the component will not have aweakness along the joint path 116.

The foregoing has described a process for bonding fiber reinforced metalcomposites using friction stir welding. While specific embodiments ofthe present invention have been described, it will be apparent to thoseskilled in the art that various modifications thereto can be madewithout departing from the spirit and scope of the invention.Accordingly, the foregoing description of the preferred embodiment ofthe invention and the best mode for practicing the invention areprovided for the purpose of illustration only and not for the purpose oflimitation, the invention being defined by the claims.

1. A method of making a fiber-reinforced component, comprising;providing a rotating friction stir welding tool comprising a cylindricalpin having a shoulder diameter of about 10.7 mm (0.420 in.), and a tipwith a length of about 2.8 mm (0.110 in.) and a diameter of about 6.4 mm(0.250 in.) affixed thereto; providing a first composite membercomprising a metal matrix with reinforcing fibers having a diameter ofabout 1 micrometer (40 microinches) to about 25 micrometers (980microinches) and a length of about 1 mm (0.004 in.) to about 38 cm (14.7in.) distributed therein in a selected orientation; providing a secondcomposite member comprising a metal matrix with reinforcing fibershaving a diameter of about 1 micrometer (40 microinches) to about 25micrometers (980 microinches) and a length of about 1 mm (0.004 in.) toabout 38 cm (14.7 in.) distributed therein in a selected orientation;and joining the first composite member to the second composite member byfriction stir welding along a predetermined joint path such that anaverage volume fraction of the reinforcing fibers within the joint pathis substantially the same after the step of joining the first compositemember to the second composite member by friction stir welding as beforethe step of joining the first composite member to the second compositemember by friction stir welding, wherein the step of joining the firstcomposite member to the second composite member by friction stir weldingis performed with a pin speed between about 700 RPM and about 900 RPM,and a traverse speed between about 10 cm/min. (4 in./min.) and about15.2 cm/min. (6 in/min.) and a force on the pin between about 499 kg(1100 lbs.) to about 635 kg (1400 lbs.).
 2. The method of claim 1wherein the reinforcing fibers are maintained in the selectedorientation within the predetermined joint path after the step ofjoining the first composite member to the second composite member byfriction stir welding.
 3. The method of claim 1 wherein the reinforcingfibers are disposed in the first composite member and the secondcomposite member in a random orientation before the step of joining thefirst composite member to the second composite member by friction stirwelding.
 4. The method of claim 3 wherein the reinforcing fibers aredisposed in a random orientation within the predetermined joint pathafter the step of joining the first composite member to the secondcomposite member by friction stir welding.
 5. The method of claim 4wherein the reinforcing fibers are disposed in a selected orientationwithin the predetermined joint path after the step of joining the firstcomposite member to the second composite member by friction stirwelding.
 6. The method of claim 1 wherein a plurality of the reinforcingfibers extend across the predetermined joint path after the step ofjoining the first composite member to the second composite member byfriction stir welding.